1. Field of the Invention
The present invention generally relates to displays made in the form of a matrix of light-emitting diodes. Such a display can be used in many devices such as cell phones, devices for taking fixed or animated pictures, audio or video walkmans or even portable computers or televisions. The present invention more specifically relates to the control of such a display.
2. Discussion of the Related Art
Electroluminescent displays are organized in the form of a matrix, elementary cells or pixels being arranged at the intersections of the lines and columns. A pixel comprises at least one light-emitting diode, for example, of organic (OLED) or polymer (PLED) type. Such diodes emit light when they are forward biased beyond a given voltage threshold and conduct a current.
Displays where the illumination control is performed by successive selection of the screen lines are considered in the present application. Such a selection is performed by selecting a line, and by having the pixels conduct a current so that the diodes are activated, that is, emit light. Generally, the luminance current is injected into the pixels from the columns. For a selected line, the number of pixels to be activated as well as their location depends on the image to be displayed, the coding of which is stored in an image memory associated with the screen control circuit. The number and the location of pixels to be activated are thus likely to vary from one line to another. Further, especially for displays in levels of grey and/or for color displays, the intensity or the duration of the luminance current injection is likely to vary.
Once the selection of a line is over, the next line is selected and the selected pixels of this next line are activated.
To accelerate the emission of the selected pixels in a line, it is desirable for these pixels to be precharged. The activation of the pixels of a line thus starts with a precharge phase.
During the precharge, each screen pixel is biased to a voltage close to the voltage that it would have if it had been active. Such a precharge then enables, on activation of the pixels of the selected line, for the current injected into the pixels to be only used for the light emission and not to charge the parasitic capacitor.
The precharge is performed either by a current control, or by a voltage precharge. In a current precharge, a constant current is injected for a very short determined duration as compared with the duration of the next light-emission phase. In a voltage precharge, a voltage is applied across the diode before entering the emission phase. For clarity, a voltage precharge is considered in the following description as an example.
U.S. patent application Ser. No. 11/294991 entitled “Automatic adaptation of the precharge voltage of an electroluminescent display” and assigned to the applicant provides automatically using the operating voltage of the previous line to perform the precharge of the selected line. Such a control enables taking into account display aging effects and the effects of the variation of the luminance current from one line to another, which would translate on the displayed image as overbrightnesses or attenuations of the brightness of a same color from one line to another.
However, despite the use of such an automated adaptation of the precharge based on the operating voltage of the previous line, displays still exhibit lines or areas with an overbrightness or an attenuated brightness.